1909.] 



The Caudal Fin of Fishes. 



13^ 



forms of the homocercal fin, in order to illustrate the process of specialisation 

 in this type, and to state very briefly conclusions as to the morphological 

 value of the endoskeletal supports of the fin-rays. 



(a) Cliipea pilchardus {Malacopterygii). — The caudal fin of this fish is takeik 

 as illustrating one of the least specialised of homocercal forms. 



Text-fig. 2. — Caudal fin of Clupea pilchardus. 

 d.c.r., dorsal caudal radial ; 

 ep., epuiul ; 

 hy., hypural ; 

 I.V., last vertebra ; 



n., notocliord ; 

 n.a., neural arch ; 

 o.c, opisthural cartilage ; 

 ur., urostyle. 



A careful reference to the above figure will render a full description; 

 unnecessary ; the points to which special attention is drawn are (1) the 

 presence of a pronounced urostyle ; (2) the presence of the notochord beyond 

 the end of the actual vertebral column ; (3) the large number of hypural 

 bones, viz., seven, associated with the last vertebral segment ; and (4) the 

 presence of an epural bone to the penultimate vertebra. A careful 

 comparison of a large number of Teleostean caudal fins has led me to regard 

 these as the features which characterise a lowly specialised homocercal 

 caudal fin. 



(b) Trigla lineata {Acanthopterygii). — The caudal fin of Trigla lineata 

 illustrates a fairly intermediate type between the lowly and highly specialised 

 homocercal form. 



